Alterations to the composition of casein micelles and retentate serum during ultrafiltration of skim milk at 10 and 40 °C
Introduction
Ultrafiltration (UF) is an important processing step in the manufacture of cheese and dairy ingredients and in milk standardisation (Gésan-Guiziou, 2013). During UF of skim milk, water, soluble minerals and lactose pass though the membrane into the permeate while proteins are concentrated in the retentate (Mistry & Maubois, 2004). Temperature is an important parameter in milk processing that affects a wide range of physicochemical properties (Fox & McSweeney, 1998). The importance of temperature on skim milk UF has been recognised since its inception, and a number of early studies investigated the effect of temperature on permeation flux rates and the microbial quality of retentates (Kapsimalis and Zall, 1981, St-Gelais et al., 1992). It was understood that low temperatures can control microbial growth but results in lower flux rates than UF performed at higher temperatures, regardless of the membrane pore size. As a result of this trade-off and to avoid temperatures most suited to microbial growth, UF of milk is performed either at or below 10 °C or at between 40 and 50 °C (Gésan-Guiziou, 2013).
Casein, which represents 80% of the protein in milk, is an important component of skim milk to be considered during UF. Casein is predominantly present in the form of hydrated colloidal assemblies known as casein micelles (CM) that consist of approximately 8% (w/w, dry matter) colloidal calcium phosphate (CCP) (Bouchoux et al., 2010, Dalgleish, 2011). The CM are responsible for the concentration polarisation and gel layer that form during UF of skim milk (Bouzid et al., 2008, Rabiller-Baudry et al., 2008, Rabiller-Baudry et al., 2005), directly influencing flux and processing efficiency.
Casein micelles are dynamic entities that can themselves be affected by UF. A fundamental understanding of casein micelle behaviour during UF could therefore lead to improvements in processing efficiency (David et al., 2008, Rabiller-Baudry et al., 2005), the use of UF retentates for standardising cheese milk, and the quality of products such as milk protein concentrate (MPC) powders. Despite this, to date only a limited number of studies have been devoted to understanding the effect of UF on the composition, size or structure of casein micelles (Singh, 2007). In one analysis of transmission electron microscopy (TEM) images a decreased average casein micelle size was observed in UF retentates at volume concentration factors of 3 and 5 compared with the unconcentrated skim milk (Srilaorkul, Ozimek, Ooraikul, Hadziyev, & Wolfe, 1991). The size changes were attributed to compositional changes in the retentate; however the composition of the micelles was not directly investigated. In another study employing TEM, the size of casein micelles was observed to remain relatively unchanged during UF but become progressively swollen on subsequent diafiltration (McKenna, 2000). In a more recent study, different methods were employed to compare casein micelles in fresh skim milk to those in commercial UF retentates and reconstituted milk protein concentrate powders (MPC). The average size of casein micelles was found to be similar in all these samples as measured by dynamic light scattering (Martin, Williams, & Dunstan, 2010). Others have investigated the effect of pH and mineral content on the physical properties of skim milk during membrane filtration and found that the casein micelle voluminosity appeared to increase in the presence of added NaCl and decrease when the pH was decreased from 6.5 to 5.5 (Karlsson et al., 2005, Rabiller-Baudry et al., 2005). In studies on the functional properties of UF-treated milk, alterations to the bulk mineral composition caused by UF have been shown to affect the renneting process (Ferrer et al., 2011, Martin et al., 2010, Sandra and Corredig, 2013). It was also found that on rennet-induced gelation of casein micelles, stiffer gels could be formed from UF concentrated skim milk than unconcentrated milk presumably due to an increase in the network bonds (McMahon et al., 1993, Sandra et al., 2011). While these studies showed that UF processing can affect the functional properties of casein micelles, alterations to the composition of the casein micelles were not investigated.
Casein micelles are affected by temperature (Davies and White, 1960, Liu et al., 2013, Rose, 1968) and these effects remain to be understood in the context of UF processing of skim milk. Of particular importance to UF processing is the exchange of calcium phosphate and casein between the micelles and the serum that is directly influenced by temperature (Davies and White, 1960, Liu et al., 2013, Rose, 1968). While the temperature of UF has been shown to affect the bulk composition and properties of UF retentates and permeates (McKenna, 2000, Pouliot et al., 1989a, Pouliot et al., 1989b, Renner and Abd El-Salam, 1991, Rose and Tessier, 1959, St-Gelais et al., 1992), a detailed study of the effects of UF temperature on the composition of casein micelles and retentate serum has yet to be performed. In addition, the effect of UF on the size and hydration of casein micelles is still not clear, and the influence of temperature on these properties has not been elucidated. The aim of this work was to investigate and compare the effect of UF concentration at different processing temperatures on the hydration, composition and size of casein micelles and the composition of the retentate serum. This information will increase our understanding of the effect of temperature on the operational behaviour of skim milk UF and the properties of the resulting retentates.
Section snippets
Skim milk samples
Samples of fresh pasteurised skim milk sourced from a local supermarket were processed and analysed within two days of purchase. Experiments were repeated as specified, using separate samples of milk. The composition of the milk as determined by external laboratory testing using standard methods (DTS Food Laboratories, Kensington, Australia) was 40 ± 1 g L−1 protein, 1.3 ± 0.1 g L−1 total fat and 47 ± 1 g L−1 carbohydrate.
Ultrafiltration
Skim milk samples were concentrated to different concentrations (volume
Results and discussion
Skim milk samples were concentrated to different concentrations by UF at 10 and 40 °C. These temperatures are at the upper and lower limits of low and high temperature ultrafiltration respectively, and were selected as it was not possible to conduct these experiments beyond these temperatures.
To investigate changes to the partitioning of milk components between the micelles and the serum during UF, retentates were fractionated by ultracentrifugation performed at the UF processing temperature of
Conclusions
During UF concentration of skim milk the progressive removal of calcium was affected by the partitioning of calcium between the micelles and the serum which was influenced by processing temperature. Performing UF at different temperatures therefore altered the final calcium content of the retentates. The composition of casein micelles including the hydration, calcium and casein content were all altered to some extent during UF and affected by the temperature of operation. These findings have
Acknowledgements
Financial support for this work was provided by Dairy Innovation Australia Limited and the Australian Research Council. The authors are grateful for the instructive comments provided by the reviewers.
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